Ha FWHM

[Rodd]

Is there a reason why FWHM for Ha would be higher than broadband FWHM?   I find that my Ha FWHM is consistently higher than my LRGB--sometimes 2x as high.  I focus the same way and in the same sky with the same equipment.

Thanks,

Rodd

[ollypenrice]

Very odd. Mine are always much lower, as you'd expect them to be since you get smaller stars through an Ha filter. I can't account for what you're finding.

Olly

[Rodd]

11 minutes ago, ollypenrice said:

Very odd. Mine are always much lower, as you'd expect them to be since you get smaller stars through an Ha filter. I can't account for what you're finding.

Olly

Well, I do usually shoot Ha during  bright Moons. Maybe that plays a part?  It probably has to do with focus, which with a b-mask is always a bit trickier with NB. Or, maybe there is something (else) wrong with my scope/camera/filters

Rodd

Edited September 15, 2019 by Rodd

[ollypenrice]

1 minute ago, Rodd said:

Well, I do usually shoot Ha during  bright Moons. Maybe that plays a part?  It probably has to do with focus, which with a b-mask is always a bit trickier with NB. Or, maybe there is something (else) wrong with my scope/camera/filters

Rodd

I must say it strikes me as suspicious. Maybe repeat the test without the moon?

Olly

[Rodd]

Just now, ollypenrice said:

I must say it strikes me as suspicious. Maybe repeat the test without the moon?

Olly

Would It be reasonable to assume that if I get a lower Fwhm with a bb filter then change to Ha and get a higher  that it is the filter and not the scope or camera?

[ollypenrice]

36 minutes ago, Rodd said:

Would It be reasonable to assume that if I get a lower Fwhm with a bb filter then change to Ha and get a higher  that it is the filter and not the scope or camera?

I'd have though so, yes. Using my 3nm Astrodon I find it very hard to get an FWHM at all since I tend to find I'm saturated on 1st mag stars and unable to generate a value on anything less! My 7nm Baaders are much easier.

Olly

[Rodd]

25 minutes ago, ollypenrice said:

I'd have though so, yes. Using my 3nm Astrodon I find it very hard to get an FWHM at all since I tend to find I'm saturated on 1st mag stars and unable to generate a value on anything less! My 7nm Baaders are much easier.

Olly

Maybe that’s it. I will look into it the next clear night. Thanks

Rodd

[Rodd]

46 minutes ago, ollypenrice said:

I'd have though so, yes. Using my 3nm Astrodon I find it very hard to get an FWHM at all since I tend to find I'm saturated on 1st mag stars and unable to generate a value on anything less! My 7nm Baaders are much easier.

Olly

I just checked--for my M82, the highest FWHM was the Ha stack (2.8 )and the lowest was the green  stack (1.8).  

[jimjam11]

Are you focusing manually or electronically? 

I have noticed that focusing by b mask gets me to rough focus with my equipment (say 4" fwhm) but if I focus via the hfr measurement in sgp I can get much lower (as low as 2.5" fwhm last night) with a ha filter.

Ha is definitely lower than broadband but the focus exposure needs to be long enough to get enough stars for sampling.

(I measured my fwhm on 5m subs in pixinsight using Moffat 4 within subframe selector).

[Rodd]

32 minutes ago, jimjam11 said:

Are you focusing manually or electronically? 

I have noticed that focusing by b mask gets me to rough focus with my equipment (say 4" fwhm) but if I focus via the hfr measurement in sgp I can get much lower (as low as 2.5" fwhm last night) with a ha filter.

Ha is definitely lower than broadband but the focus exposure needs to be long enough to get enough stars for sampling.

(I measured my fwhm on 5m subs in pixinsight using Moffat 4 within subframe selector).

I focus with a b mask. It doesn’t matter. Why can I achieve excellent focus in all other filters except Ha. I can focus oiii and sii just fine. On good nights I can achieve  Fwhm of less than 2. In general, focus is not the weak point in my images.   Besides, it wasn’t always this way.  Maybe an insect has left a web or fine filaments on my Ha filter? I had a spider in my cooling fans once that was webby 

Rodd

[jimjam11]

31 minutes ago, Rodd said:

I focus with a b mask. It doesn’t matter. Why can I achieve excellent focus in all other filters except Ha. I can focus oiii and sii just fine. On good nights I can achieve  Fwhm of less than 2. In general, focus is not the weak point in my images.   Besides, it wasn’t always this way.  Maybe an insect has left a web or fine filaments on my Ha filter? I had a spider in my cooling fans once that was webby 

Rodd

I need to work on my b mask focusing in that case!

Sounds like you need to check the ha filter in that case, have you ever tried measuring fwhm in different parts of the frame,  such as with the pixinsight fwhmeccentricity script? If it was something on the filter you would expect differences across the frame?

[Rodd]

24 minutes ago, jimjam11 said:

I need to work on my b mask focusing in that case!

Sounds like you need to check the ha filter in that case, have you ever tried measuring fwhm in different parts of the frame,  such as with the pixinsight fwhmeccentricity script? If it was something on the filter you would expect differences across the frame?

That is a good idea.  I have not used that script yet.  Tonight it is supposed to be clear.  The Moon is up after 9 so I will have to image with Ha--but before that I will take a sub with a broadband filter then with the Ha filter.  I will trty and get to the bottom of it.  Meanwhile I will use the script and see.  Its probably not a physical thing though--unless the filter is degrading somehow...if that can even happen.  

Rodd

[retret66]

I have starlight boss II focuser and SGP will find focus just fine but takes time like 10min depending on how strict I set it to. But I find it very easy just magnifiying 1:1 pixel and manually adjusting the focus via its software is quicker and just as good. The FWHM on NB should be lower than broadband period. try like a 5sec exposures to rule out mount tracking issues on longer exposures.

Edited September 16, 2019 by retret66
coreection

[Rodd]

19 minutes ago, retret66 said:

I have starlight boss II focuser and SGP will find focus just fine but takes time like 10min depending on how strict I set it to. But I find it very easy just magnifiying 1:1 pixel and manually adjusting the focus via its software is quicker and just as good. The FWHM on NB should be lower than broadband period. try like a 5sec exposures to rule out mount tracking issues on longer exposures.

Well-I rule that out by choosing a bright star in my FOV and focusing on that while the guider is working.  That way I can keep guiding, keep my framing, focus and save time.  It works great--if there is a bright star in the field.  If not, I have gone as far as 60 sec exposures (very rarely).  For Ha its usually 10-15 sec.  For Broadband its much shorter--less than mu guide exposure length of 4 sec.  .  Maybe that is the cause though-guider movement ruining focus....never thought of that.  BUT....I start out the night focusing on a bright focus star before slewing to my target--usually one close by.  This is a star that gives great response to the B-Mask (1-4 mag star), and the FWHM is no better.  So I wonder if it IS the cause.  Right now I am giddy--I am supposed to get 5 consecutive days of clear nights!  If only I had confidence in the forecast.  Should be plenty of time for me to try and get to the bottom of it. 

Rodd

[retret66]

11 minutes ago, Rodd said:

Well-I rule that out by choosing a bright star in my FOV and focusing on that while the guider is working.  That way I can keep guiding, keep my framing, focus and save time.  It works great--if there is a bright star in the field.  If not, I have gone as far as 60 sec exposures (very rarely).  For Ha its usually 10-15 sec.  For Broadband its much shorter--less than mu guide exposure length of 4 sec.  .  Maybe that is the cause though-guider movement ruining focus....never thought of that.  BUT....I start out the night focusing on a bright focus star before slewing to my target--usually one close by.  This is a star that gives great response to the B-Mask (1-4 mag star), and the FWHM is no better.  So I wonder if it IS the cause.  Right now I am giddy--I am supposed to get 5 consecutive days of clear nights!  If only I had confidence in the forecast.  Should be plenty of time for me to try and get to the bottom of it. 

Rodd

Good Luck!! I hope you will solve your issue. I use APT, and I use 1:1 preview and the lowest exposure I can use. Then there is a focus aid which measures the FWHM as you adjust. I just use 5sec exposures and magnify the faintest star I can find and so far this works a lot faster than autofocusing in SGP.. The FWHM will change depending on guiding it will just add error to the FWHM calculation.

[Rodd]

23 minutes ago, retret66 said:

Good Luck!! I hope you will solve your issue. I use APT, and I use 1:1 preview and the lowest exposure I can use. Then there is a focus aid which measures the FWHM as you adjust. I just use 5sec exposures and magnify the faintest star I can find and so far this works a lot faster than autofocusing in SGP.. The FWHM will change depending on guiding it will just add error to the FWHM calculation.

Thanks--I just find it odd.  With broad band I might have a FWHM of lets say 2.0 and then a sub will come out and be calculated at 2.6--time to refocus, and the central spike will be noticeably off-center.  I can easily see changes in FWHM of .5 with a B-Mask.  Even less. .3.  So when the spike is centered for the lum filter and I get 2.1 and it is centered for Ha and I get 3.2--something is indeed wrong.

Rodd

 

[Rodd]

1 hour ago, retret66 said:

Good Luck!! I hope you will solve your issue. I use APT, and I use 1:1 preview and the lowest exposure I can use. Then there is a focus aid which measures the FWHM as you adjust. I just use 5sec exposures and magnify the faintest star I can find and so far this works a lot faster than autofocusing in SGP.. The FWHM will change depending on guiding it will just add error to the FWHM calculation.

I just remembered about Bhatinov Grabber.  Its free software that assists with B-mask focus.  The software studies a sub of the focus star and tells you when it is in the proper spot.  Not sure how good it works

Rod

[retret66]

I have that also but with bathinov mask although the diffraction is good, focus is still slightly off so I never use it

[Tim]

9 minutes ago, retret66 said:

I have that also but with bathinov mask although the diffraction is good, focus is still slightly off so I never use it

Used properly a Bahtinov mask is pretty much foolproof, are you manually adjusting focus or automating it to get it slightly off?

 

[Rodd]

24 minutes ago, retret66 said:

I have that also but with bathinov mask although the diffraction is good, focus is still slightly off so I never use it

I don't use the grabber--I do not know if it is any good.  i like the Idea, but that doesn't mean it works well.  I would like to try it....but I am averse to learning new things!

Rodd

[Rodd]

I guess 2.5 isn't too bad--it just rankles me every time I shoot with Ha it is higher than RGB and L.  Last night I was very careful--focusing and refocusing.  The same method I use for broadband.  There should be no difference.  I chose a bright star--centered that spike.  stayed on the sta5r for 35 min as the temp equalized and kept tweaking focus to keep spike centered.  the spike was centered and Maxim reported the FWHM numbers for each sub and I stayed on the lowest I could achieve.   What was the FWHM of my first sub?  2.75.  I do the same for green or red and its 1.9 or 2.1.  Could it be the camera?  Could it be Pixinsight not reporting an accurate FWHM?  Not sure it matters--I am shooting the Wall with with a 5' scope at 1,000mm and comparing the stacks to an image shot with a widefield setup.  the FWHM values are similar--but I am getting much better resolution of double stars (or stars that look like double stars).  Definite separation with the higher res subs while the double star looks like a bloated single star in the widefield image.  It is all very confusing

Rodd

[jimjam11]

5 minutes ago, Rodd said:

I guess 2.5 isn't too bad--it just rankles me every time I shoot with Ha it is higher than RGB and L.  Last night I was very careful--focusing and refocusing.  The same method I use for broadband.  There should be no difference.  I chose a bright star--centered that spike.  stayed on the sta5r for 35 min as the temp equalized and kept tweaking focus to keep spike centered.  the spike was centered and Maxim reported the FWHM numbers for each sub and I stayed on the lowest I could achieve.   What was the FWHM of my first sub?  2.75.  I do the same for green or red and its 1.9 or 2.1.  Could it be the camera?  Could it be Pixinsight not reporting an accurate FWHM?  Not sure it matters--I am shooting the Wall with with a 5' scope at 1,000mm and comparing the stacks to an image shot with a widefield setup.  the FWHM values are similar--but I am getting much better resolution of double stars (or stars that look like double stars).  Definite separation with the higher res subs while the double star looks like a bloated single star in the widefield image.  It is all very confusing

Rodd

You have definitely inspired me to try and improve my b mask focussing, HFR focussing in SGPro is time consuming (but I have found it very accurate).

Did you manage to inspect the filter, although I am guessing the difference between 2.1" and 2.75" is large in measurement terms but miniscule in actual terms?

[Rodd]

7 minutes ago, jimjam11 said:

You have definitely inspired me to try and improve my b mask focussing, HFR focussing in SGPro is time consuming (but I have found it very accurate).

Did you manage to inspect the filter, although I am guessing the difference between 2.1" and 2.75" is large in measurement terms but miniscule in actual terms?

I didn't have time--well, I had time but all I could think about was collecting data!  In reality--When I process all my subs--say 120 with 20-25 subs with elevated FWHM, a few in the 5s a few in the 4s and compare that with a stack of 100 subs where I threw out any sub with a FWHM over 3.2 or 3, there is virtually no difference.  In fact, the FWHM might be .1 or .2 higher, but the SNR is much higher and the noise lower.  But--on the other hand, if I only achieve a 3.5-4.0 FWHM for all my subs, the stack is definitely soft.  In reality, for my setup (TOA 130 with asi 1600) the resolution is .78 arcsec/pix...way too low (I mean high) for my sky really.  A perfect FWHM I am told is 1.6x the resolution--so in my case it would be about 1.25.  My sky is never that good so its the limiting factor.  When I get down to 1.8 or 1.9 I am really spot on.  At that point its not only focus that's critical but guiding as well.  My guide resolution is a tad high for this setup and may be introducing a few tenths increase in FWHM, maybe .5.  So when I get 2.5-2.7--really anything less than 3 the subs are definitely usable.  Its just a mystery why Ha is higher than the other filters.  I thought it might be the B-mask not being straight--but it works for the other filters.  I thought it might be the magnitude of the focus star--but I have used 1st mag stars and I get the same value.  Maybe there is a film on the Ha filter.  The flats look a lot cleaner than the Lum or red or green though.   Its hard to see looking down the scope--I will have to take the camera off and open up the filter wheel and inspect the filters.  that will happen when I change OTAs--which I want to do soon--as soon as I finish the images I am working on.  The problem is the Moon will come out and I will have to shoot Ha and that means starting a new image....then i have to finish it.  Sort of a never ending procession.

Rodd

[Adam J]

On 09/09/2019 at 14:10, Rodd said:

Is there a reason why FWHM for Ha would be higher than broadband FWHM?   I find that my Ha FWHM is consistently higher than my LRGB--sometimes 2x as high.  I focus the same way and in the same sky with the same equipment.

Thanks,

Rodd

The only reason I can think that something like that would happend is if you are doing longer subs and so getting some flex or guiding errors that are making the stars bigger in comparison to the shorter LRGB subs. Apart from that there is no physical reason I know if why that would happen. The other reason might be that your not detecting any lower brighness stars at all due to insufficient exposure and a very narrow filter. I have often noted that smaller aperture scopes tend to have a cut of in Ha in terms of star size. 

Edited September 17, 2019 by Adam J

[Rodd]

14 minutes ago, Adam J said:

The only reason I can think that something like that would happend is if you are doing longer subs and so getting some flex or guiding errors that are making the stars bigger in comparison to the shorter LRGB subs. Apart from that there is no physical reason I know if why that would happen. The other reason might be that your not detecting any lower brighness stars at all due to insufficient exposure and a very narrow filter. I have often noted that smaller aperture scopes tend to have a cut of in Ha in terms of star size. 

Hey...Your a genius--of course!  My Ha subs are much longer than my other subs.  Well--with CCD 3-4x longer.  With Cmos about 2.5 times longer.  But still.  that makes sense.

Rodd

EDIT--Both reasons are really good points--Thanks

Edited September 17, 2019 by Rodd